Systems for scrambling a television data stream are well-known in the art. One such system is described in the following U.S. Pat. No. 5,282,249 to Cohen et al.; U.S. Pat. No. 5,481,609 to Cohen et al. Scrambled television data streams described in the Cohen et al. patents comprise both scrambled data representing television signals and coded control messages, also known as ECMs. The ECMs of Cohen et al. comprise, in a coded form, data necessary for generating a control word (CW) which may be used to descramble the scrambled data representing television signals.
While the two patents to Cohen et al. describe an analog system, that is, a system in which analog television data streams are broadcast to television sets, it is appreciated that similar ECM methods may also be used for digital television data streams. Generally, the scrambling techniques used for scrambling analog television signals such as, for example, the well-known "cut-and-rotate" technique, are chosen for their applicability to analog signals. In scrambling of digital television signals other scrambling techniques, well-known in the art, are used, the techniques being more appropriate to digital signals such as, for example, applying the well-known DES algorithm to the digital television signals.
Methods of transmitting a scrambled digital signal, including ECMs, are described in the MPEG-2 standard, ISO/TEC 13818-6, Jul. 12, 1996 and subsequent editions.
Recording of analog television signals such as, for example, recording using a VCR, is well-known in the art and VCR equipment is widely commercially available from a variety of manufacturers. Recording of digital television signals is also known. A consumer digital VCR is described, for example, in the article "A Consumer Digital VCR for Digital Broadcasting" by Okamoto et al., published in IEEE Transactions on Consumer Electronics, Vol. 41, No. 3, August 1995, pp. 643-649.
U.S. Pat. No. 5,574,787 to Ryan describes an apparatus and method for copy protection for video platforms in which a specially adapted video recorder, playback device, or set top decoder is used to protect copyright material.
European patent application EP 0 714 204 A2, assigned to LG Electronics, Inc., describes a method for copy protection in digital video systems.
The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.